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Dr.Ehsan

DC Power Supply (5V 10A) - Questions

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Hello,
I'm planning to design a circuit for a DC Motor with these parts:
1* 7805
2 * 2N2955 (paralleling two transistors to increase current)
plus some capacitor and resistors.
(the circuit image is attached)
Here is my question:
If I use a 12volts-6Ampere transformer ( 12V * 6A = 72W), does it provide enough power to have 5V-10A as output?
theoretically: 72W/5V = 14.4Ampere


Thanks,

post-75465-14279144415802_thumb.jpg

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The 12V transformer has a peak voltage of 1.414 x 12= 17V. The rectifiers operate at the 17V. The max continuous power from the transformer is 72VA so the max continuous DC current from this power supply is 72/17= 4.2A.

17V is much too high for a 5V supply because the extra 12V simply wastes power causing a lot of heat. Use an 8V transformer instead. The current rating must be high enough to power the motor plus heat the circuit. 8V has a peak voltage of 11.3V. The rectifiers will drop it to make +9.3V and also use transformer power creating heat. The transistors also use transformer power creating heat.

Your circuit has 3 resistors in parallel which can be a single resistor. Each transistor should have a low value series emitter resistor so that the transistors share the load current. 

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There's a simple switching circuit (including LM2576), which has 5V - 3A output.
(the circuit image is attached)
Datasheet: http://www.national.com/ds/LM/LM2576.pdf
Question: Is it possible to increase its current by power transistors?





post-75465-14279144415859_thumb.jpg

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A DC motor doesn't need a regulated power supply.

Buy a 6V transformer with the desired current rating, connect it to a bridge rectifier and run the motor from that.

If you need a smooth DC power supply to run a microcontroller you can use a voltage doubler circuit and the LM7805 to dereive a separate, smooth 5V supply.

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I use a PC SMPS for all my high current projects that need a 5V supply at a high current rating. The SMPS rails are shortcircuit protected at each voltage, there can be some high frequency ripple but you can use additional filter caps for it (by the way the maximum ripple stipulated by the design specs is 1% of full voltage; i measured a ripple of 3-4mV in a 5V supply) to switch on a PC smps just connect the power On wire (green wire) to ground (black wire). the supply is in standby and when the green wire is open or connected to +5V.

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